Run this query in the SQL Box at phpmyadmin. (Choose the db where your forum resides on).

Code:

SELECT u.username, u.email, COUNT( * ) AS posts
FROM xf_post AS p
LEFT JOIN xf_thread AS t ON ( t.thread_id = p.thread_id )
LEFT JOIN xf_user AS u ON ( u.user_id = p.user_id )
WHERE t.node_id IN(x,y)
GROUP BY p.user_id
ORDER BY posts DESC
;

Replace x and y with the actual node id's that you want to pull the info. Also if you are using another prefix, other than the standard xf_, for your forum tables, then use that instead appended to the table names.